The National Transportation Safety Board issued findings and recommendations after a probe into a 2016 crash near St. Marks, Florida, that left four dead, and dozens more injured after a bus carrying migrant workers ran a stop sign and was struck by a tractor-trailer.

The 56-year-old driver of the farmworkers' bus was likely suffering from sleep deprivation, the board found. It added the driver missed signs at an intersection where roadwork was being carried out that an "alert and attentive" driver would have recognized.

The owner of the bus, Billy R. Evans Harvesting, Inc., also failed to properly oversee its drivers and vehicle safety, the NTSB's report said. The company has since been ordered to cease all operations following the crash, according to the Federal Motor Carrier Safety Administration.

The board also looked into two other crashes: a 2015 accident outside Little Rock, Arkansas, that killed six migrant workers and another accident that killed six in Virginia.

The NTSB found that drowsy driving was a factor in all three crashes.

An Associated Press review found more than a dozen similar crashes that left at least 38 dead and nearly 200 injured just since January 2015, a tally that included these three accidents. But no one knows for sure how many there are; no agency keeps track of casualties nationally.

The NTSB also found that, despite the existence of federal and state safety rules meant to protect them, farmworkers are still transported in unsafe buses and vans.

"Motor carriers and farm labour contractors often transport workers in unsafe vehicles and without properly qualified and rested drivers," the NTSB report states. "Federal and state agencies have been ineffective in deterring unsafe operations."

The NTSB specifically highlighted the Federal Motor Carrier Safety Administration and the Department of Labor, the agencies charged with enforcing the regulations.

Duane DeBruyne, an FMCSA spokesman, said in an email that the "NTSB is a vital partner in transportation safety and we will thoroughly review the findings of their investigation." The Department of Labor did not immediately respond to an email seeking comment.

The NTSB found that both agencies failed to adequately oversee Billy R. Evans Harvesting prior to the Florida crash, likely due to being understaffed and not having sufficiently trained investigators.

Among its recommendations, the board said the two agencies need to work out a system to share safety data with each other, and to develop new data-driven approaches for targeting the enforcement of existing safety rules given their staffing issues.

OPINION The global food system is amazing. We have created a process that can put enormous diversity of food onto any plate in our province, assuming the plate will be sliding in front of someone who can afford to pay